This invention relates to a pressure vessel and more particularly to an improved filter cloth for a leaf filter used in such a pressure vessel.
A pressure vessel is a common type of pressure filter which is utilized in various industries, including the manufacture of chemicals, paints and many pharmaceutical preparations. A pressure vessel is arranged with its axis either vertical or horizontal, where filter elements inside such a vessel may be either flat leaves or tubes, and may be assembled with their surface either horizontal or vertical. A cake is formed within the pressure vessel from the solid material which has been extracted from the filtered liquid.
The choice between a vertical and a horizontal filtering surface must take into account two conflicting factors, both of which arise from the fact that cake has a natural tendency to fall off either a vertical surface or the underside of a horizontal one. One the one hand, this tendency aids discharge of the cake at the end of a cycle; on the other, however, it presents the danger of cake falling off at a premature stage, notably when operation is interrupted for any reason, or when unfiltered slurry is being displaced from the vessel and wash liquor introduced. In practice, therefore, the horizontal leaf filters only through its upper surface, which means that half the surface is wasted and thus the cost of the filter increased; the construction is therefore generally limited to areas of less than some 600 sq. ft., but finds particular use where thorough washing is required.
In a horizontal vessel, such as a Sweetland filter, the leaves are vertically mounted, with the space between adjacent leaves being usually chosen to suit the cake thickness for a particular duty. The leaves are circular, static and each provided with its own independent filtrate outlet located at the top leading to an external common manifold pipe. Cake discharge is either wet, with suitable sluice nozzles, or day, when bottom of the filter is swung open.
A filter cloth is placed about each leaf. The slurry or other liquid to be filtered is introduced into the hollow portions of the vessel and passes through the filter cloth into the leaf. The filtered liquid is passed through the outlet contained within the filter cloth. As the process continues, the hollow space gradually fills with solid cake material from which the filtered liquid has been extracted. At regular intervals, the press is opened, the leaves separated, and the cake removed. The filter cloth must also be cleaned and replaced again or substituted by other filter cloths.
The filter cloth surrounding the leaf filter must be held in a water tight connection surrounding the leaf filter. To achieve this, some filter cloths surround three sides of the leaf filter in a U-shaped configuration with their open ends extending into the vessel frame itself so as to be held in place by the force of the frame against the leaf filter. However, while these filter cloths provide a tight fit around the leaf, they do not provide a sufficient water tight seal around the outlet opening from the leaf filter.
Other filter cloths are formed to completely surround the leaf filter on all sides except for the single opening in the filter cloth through which the outlet opening can pass. While these filter cloths may provide sufficient sealing around the filter plates by sewing or other permanent enclosing means, they are difficult to remove from the leaf filter for cleaning and replacement. In these latter types of filter cloths, it is necessary to either destroy the filter cloth by cutting in order to remove it from the leaf filter, or to remove the stitches and subsequently restitch and reseal the entire cloth around the leaf filter.
Another problem with the filter cloths heretofore used relates to the pattern blank from which they are formed. In most cases, the filter cloth is flat, since the leaf filter which it surrounds is of a thin flat shape. The filter cloth is usually formed of two identically shaped blanks which are separated and which are sewn together around their perimeters to form a complete enclosure which can fit around the leaf filter. However, in forming the enclosure a section is left unsewn to accommodate the outlet. Since the sections adjacent to the outlet opening are sewn or sealed together from two separate blanks, there is generally insufficient sealing provided in the immediate vicinity of the outlet opening.